It is well known in the field of hand operated dispensing pumps to utilize a resiliently deformable bulbous diaphragm which cooperates with the pump housing to define a pump chamber. Such a diaphragm typically performs three valving functions, namely: controlling inflow of fluid from a container to the pump chamber; controlling outflow of fluid from the pump chamber; and venting the space in the container above the fluid contents during inflow of fluid to the pump chamber. Inflow and outflow of fluid to and from the pump chamber is generally controlled by lip or flange portions of the diaphragm which cooperate with passages in the pump housing in a flapper valve configuration. Venting control is sometimes achieved by positioning the main bulbous diaphragm body portion to close a vent passage in the pump housing except during pump operation, although alternative venting schemes have been used. Examples of this type of prior art pump may be found in U.S. Pat. Nos. 3,726,442 (Davidson et al), 3,986,644 (Grogan et al), 3,987,938 (Cooprider et al), 3,995,774 (Cooprider et al), 4,199,083 (LoMaglio).
Pumps of the type described above suffer from certain practical disadvantages. One such disadvantages resides in the fact that the outlet flapper valve closure, when the pump is in use, is formed by a diaphragm lip or flange in a relaxed or unstressed state. The closure is therefore not positively sealed and leakage from the pump chamber is possible, particularly if the container is tipped over and, more particularly, in the case of low viscosity fluids. A further disadvantage of prior art pumps of this type resides in the fact that a considerable portion of the fluid in the pump chamber is not expelled during a chamber compression stroke, primarily because of the difficulty and/or inconvenience involved in fully compressing the diaphragm. Another disadvantage of these prior art pumps is that the diaphragm is exposed to the ambient environment and thereby subject to inadvertent rupture if the pump/container assembly is carelessly handled or mishandled. A still further disadvantage relates to the fact that these prior art pumps generally require six independent parts to be fabricated and assembled to form the pump. It is desirable to reduce the number of parts in order to reduce the cost and assembly complexity of the pump; in fact, an attempt at reducing the number of parts is found in the aforesaid Cooprider et al U.S. Pat. No. 3,987,938. However, part reduction is achieved in that patent at the expense of increased assembly complexity and cost (i.e. it is necessary to perform a swaging operation to secure the diaphragm in place) and parts complexity (i.e. the diaphragm is provided with three independently depending annular flanges of different diameter).